Modern biotechnology has devoted considerable effort to the development of phenotypically distinct plants with economically advantageous qualities. Valuable features in food crops include increased yields, extended shelf-life, and delayed fruit ripening that is susceptible to external control. In the floral industry, there is interest in delaying senescence of both cut and uncut flowers.
Efforts to develop crop plants that produce higher yields have been directed toward pest control or toward the selection and breeding of varieties that bear greater numbers of fruits, or that produce larger fruits. These crop breeding endeavors are very time-consuming and labor-intensive, and have not resulted in dramatically increased crop yields.
Much of the research on senescence in plants has focused on the manipulation of the plant hormone cytokinin, because there is evidence that suggests an inverse correlation between levels of the plant hormone cytokinin and the onset of senescence. Plant varieties with high levels of endogenous cytokinin tend to have blooms that are longer lived. The application of cytokinin to blooms or to the holding solution of cut flowers has been tested as a means for extending flower longevity. The success of this method is equivocal, and plant response to cytokinins is affected by numerous parameters, some of which are immutable.
One of the means by which cytokinin is thought to delay floral senescence is by decreasing the sensitivity of floral tissues to ethylene and/or interfering with the production of ethylene. Increased levels of ethylene are correlated with accelerated senescence in petals. Experiments designed to manipulate ethylene levels were conducted using transgenic carnations that contained a construct directing expression of an antisense RNA complementary to the mRNA of ACC synthase, an enzyme involved in the biosynthesis of ethylene. The results of that research did not conclusively demonstrate delayed senescence in flowers of transgenic carnations in which the antisense RNA was expressed.
In fruits, high levels of cytokinins are associated with delayed ripening, but not delayed senescence. The exogenous application of cytokinins to ripening fruit has been employed to delay ripening. U.S. Pat. No. 5,177,307 describes the manipulation of cytokinins in transgenic tomato plants containing a construct that directs the tissue-specific expression of an enzyme involved in the biosynthesis of cytokinin. These transgenic tomato plants exhibit increased expression of cytokinins, and produce fruit with a blotchy appearance.
Tillable land available for production of food crops continues to diminish because each year, more acreage is devoted to alternative uses. At the same time, the human population is rapidly increasing. Therefore, it is essential to increase agricultural productivity to meet the nutritional needs of the world's burgeoning population.
Within the floral and landscaping industries, producers, florists, and professional gardeners and landscapers are desirous of methods for increasing the number and persistance of blooms on ornamental flowering plants and cut flowers. Human enjoyment of ornamental flowering plants and cut flowers can be enhanced by extending the longevity of the flowers.